High-vacuum diffusion pump



May 2, 1950 E. w. FLOSDORF HIGH-VACUUM DIFFUSION PUMP Filed July 30, 1947 INVENTOR. EARL W. FLOS DORF' Q W m/ I ATTORNEYS Patented May 2, 1950 HIGH-VACUUM DIFFUSION PUMP Earl W. Flosdorf, Forest Grove,

Pm, assignor to F. J. Stokes Machine Company, a corporation of Pennsylvania Application July-30, 1947, Serial No. 764,862

I] 'This invention relates tohigh vacuum pumps of the. type inwhich" mercury, oeothur pump fluid having a low vapor pressurefls'vapor-i 8 Claims. (cl. 236-401) ized by heat and the vapors passthrougha chirp- '1 ney in the pump casing to an umbrella jet asso-.

ciated with the chimney which directs the vapors. a

in the reverse direction into the annular space in the pump casin around the chimney so that the gas from the chamber being evacuated is diffused into the stream of vapors leaving the jet.

A difiusion pump of this type is shown inthe patent to Embree, No. 2,150,676, issued March 14,;

1939. With the object of increasing the pumping speed, the inner wall of the high vacuum umbrella jet of the Embree pump is made to slope inwardly away from the inner surface of the pump. casing. This is accomplished by shaping the chimney so that it has a constricted portion in the vicinity of the umbrella jet, thus causing the chimney wall to slope inwardly and downwardly away from the jet. This is said to allow the vapors issuing from the jet to expand inwardly toward the center axis of the pump and to take a general downward direction in the pump casing instead of spreading laterally when they leave the jet.

I have ascertained that a difiusion pump of the type shown in the Embree patent will pump at a still higher speed, if the inner wall of the high vacuum umbrella jet does not slope inwardly as taught by the Embree patent but is parallel to the inner surface of the pump casing, and if the inner surface of the chimney nevertheless slopes outwardly as it approaches the jet. By combining these two features in a difiusion pump a very high pumping speed can be obtained.

The principal object of the invention, therefore, 'is to improve a diffusion pump of the type shown in the Embree patent so that it will pump at higher speed.

A pump embodying the invention is illustrated in the accompanying drawing, in which:

Figure 1 is a vertical section through the pump;

Fig. 2 is a duplication of that portion of Fig. l which illustrates the high vacuum umbrella jet drawn to a larger scale; and

Fig. 3 is an inside view of the cap or umbrella which fits on the top of the vapor tube.

Referring first to Fig. 1, a three-stage difiusion pump is illustrated therein which is similar to the three-stage diffusion pump shown in the Embree patent except for the construction of the third stage or high vacuum umbrella jet. The pump comprises a cylindrical pump casing l; which may be made of glass or metal, having a water jacket l a. The pump casing is adapted to up of four sections 4,- 5,

c i v 2" I beoonnected-atits upper end 2 to the chamber tube: evacuated; AFchimney, denoted in general reference numeral "3, is. positioned concentrically in-the pump casing. The chimney i made 6" and T, which form a vapor-conducting tube, and also the other associated parts hereinafter described. All parts of the chimney may be made of aluminum, stainless steel, or other suitable metal. The section 4 ol' the chimney is an enlarged base portion which fits with a small clearance within the lower portion-'of'the pump casing. Any suitable pump fluid having a low vapor pressure, such as di-amyl phthalate, is introduced into the bottom of the pump casing through an inlet 8 and occupies a part of the space within the enlarged base portion 4 of the chimney as shown at 9. The pump fluid is heated to vaporize it by means of an electric heater Ill.

The first-stage jet is formed by providing the chimney with a circular series of openings II and by securing to the outside of the chimney a pair of skirts l2 and I3, the skirt l2 being secured to the chimney just above the openings II and the skirt l3 being secured to it just below the openings, and the two skirts having walls which slope outwardly from top to bottom in parallel relation so that vapors leaving the chimney through the openings l I are directed downwardly and outwardly by the skirts toward the inner surface of the pump casing in the form of an annular jet. The openings H in the chimney may be conveniently formed by spacing the two sections 5 and 6 of the chimney apart by means: of a I number of spacer strips I4. I

The second-stage jet may be conveniently formed as follows: The upper edge of the chimney section 8 is turned inwardly to form a flange I! having an opening I6 through which vapors can pass from the chimney section 6 into the upper chimney section I. The lower portion of the upper chimney section 1 is conical in shape and is adapted to telescope a small distance over the upper end of the chimney section 6. A series of nuts l1 attached to the inner surface of the chimney section i not far from its lower edge support the chimney section 1 on the curved upper edge of the chimney section 6. The nuts I! serve as spacers and space the lower end of the wall of chimney section i a short distance away from. the wall of chimney section 6; thereby providing a series of openings between these walls and between the nuts I? through which vapors from the interior of the chimney can pass downwardly into the annular space in the pump casing.

The first and second-stage jets just described.

although structurally somewhat different from the first and second-stage jets of the pump disclosed in the Embree patent, operate on the same principle, the first-stage jet being capable of operating against a high back pressure and creating a lower back pressure for the second-stage jet to operate against, the two stages in conjunction producing a low back pressure for the third stage jet now to be described.

As previously indicated, the third stage jet or high vacuum umbrella jet located at the top of the chimney 3 is constructed differently than taught by the Embree patent. The walls of the upper chimney section 1 taper to a throat l8 located just below the third stage or high vacuum umbrella jet. Above the throat IS the walls of the chimney section I flare outwardly. The upper end of the chimney section I is open except for a cap or so-called umbrella l9 supported on the chimney section in spaced relation to its upper edge as hereinafter described. The cap has a downwardly extending peripheral flange 20 which flares outwardly toward the inner surface of the pump casing. The cap I9 is supported on the upper edge of the chimney section I by three or more right-angle clips 2| (Fig. 2). One arm of each of the clips is secured, as by welding, to the inner surface of the cap and the other arm extends downwardly over the edge of the chimney section 1 as shown at 22. The clips 2i are so positioned that their downwardly extending arms 22 telescope over the upper edge of the chimney section I with a snug fit. Fig. 3 shows the clips as they appear when the cap I9 is removed from the chimney and is viewed toward its interior.

The horizontal arms of the clips rest on the upper edge of the chimney section I and therefore a space is provided all around the upper edge of the chimney, except where the clips themselves are located, through which vapors can pass from the chimney into the cap and be deflected downwardly by the caps peripheral flange 20, as indicated by the arrows in the drawing. The inner surface of the cap l9 also has a boss 23 (Figs. 2 and 3), and'this boss and the top wall of the cap are tapped as shown at 24 in Fig. 3 to receive a bolt 25 (Fig. 2) by means of which the cap is secured to a spider 26. This spider functions, in conjunction with the'enlarged portion 4 at the base of the chimney, to hold the entire chimney in concentric relation to the pump casing l.

Around the constricted portion of the chimney section I which forms the throat there is positioned a cylindrical sleeve 21 which may be held in position by tack welding it to the section I, or in any other suitable way. The diameter of the external surface of the sleeve 21 is about the same as the external diameter of the upper edge of the chimney section I. The cylindrical sleeve extends upwardly into close proximity with the upper edge of the chimney section I and telescopes under the downwardly extending portions 22 of the cap clips 2|.

It will thus be seen that the upper end of the chimney is double-walled, the inner wall being the wall of the chimney section 1 and the outer wall being formed by the cylindrical sleeve 21. The inner wall slopes outwardly from the throat l8 as it approaches the entrance to the umbrella jet, i. e. the clearance around the upper edge of the chimney through which the vapors pass from the chimney into the outside space in the pump casing. The inner surface of the downwardly extending flange 20 of the cap l9 constitutes the outer wall of the high vacuum umbrella jet. The

outer surface of the sleeve 21 constitutes the inner wall of this jet. Thus the inner wall of the jet is straight and is parallel to the inner surface of the pump casing I, but the inner surface of the chimney along which the vapors flow from the throat I8 to the entrance of the jet slopes outwardly toward the jet.

The gases pumped by the vapors leaving the three jets pass into a discharge conduit 28. The vapors of the pump fluid that are not already condensed on the walls of the pump casing l, cooled by the water jacket la, pass to the conduit 28 along with the pumped gases and are condensed in a water-cooled condenser 29. the condensed vapors return to the body of pump fluid in the lower end of the pump casing by flowing downwardly through the small space between the enlarged lower portion 4 of the chimney and .the vapors in passing to this jet from the throat l8 move along an inner surface of the chimney which slopes outwardly toward the jet, 1. e., slopes outwardly as it approaches the entrance to the jet. This has been found to be important since the results are not nearly as satisfactory if the inner surface of the chimney section I has parallel sides. The outwardly sloping inner surface of the chimney is preferably a curved surface as shown in the drawing.

A test under identical conditions of two pumps of the kind herein described and identical in all respects except that the high vacuum umbrella jet of the first pump was constructed in accordance with the teachings of the Embree patent, whereas the high vacuum umbrella jet of the second pump was constructed as herein described, showed a pumping speed of the second pump that was nearly two and one-half times greater than that of the first pump at a pressure of .0002 mm. The improved performance of the second pump was therefore due entirely to the fact that the high vacuum umbrella jet was constructed in accordance with the present invention.

I claim:

1. In a vacuum diffusion pump having a casing and a vapor-conducting chimney therein provided with an umbrella jet, the improvement in which the chimney is constructed and arranged to provide an inner jet wall which is substantially parallel to the inner surface of the pump casing and an inner chimney surface which in the vicinity of the jet slopes outwardly and upwardly toward the jet.

2. In a vacuum diffusion pump comprising a casing and a vapor-conducting chimney therein provided with an umbrella jet having an entrance through which vapor from the interior of the chimney passes into the jet, the improvement in which the chimney is constructed and arranged to provide an inner jet wall which is substantially parallel to the inner surface of the pump casing and an inner chimney surface which slopes outwardly and upwardly as it approaches the jet entrance.

3. In a vacuum diffusion pump comprising a casing and a, vapor-conducting chimney therein provided with an umbrella jet having an entrance through which vapor from the interior of the chimney passes into the jet, the improvement All of 'g in which the chimney is constructed and arranged to provide an inner jet wall which is substantially parallel to the inner surface of the pump casing and an inner chimney surface which curves outwardly and upwardly as it approaches the jet entrance.

4. In a vacuum diffusion pump having a casing and a vapor-conducting tube therein provided with an umbrella jet and whose wall in the vicinity ofthe jet slopes outwardly toward the jet, the improvement which comprises a sleeve surrounding the outwardly sloping portion of the vapor tube wall and whose outer surface is substantially parallel to the inner surface of the pump casing and forms the inner wall of the umbrella jet.

5. A vacuum difl'usion pump comprising a casing, a vapor-conducting tube therein provided with an umbrella jet, said tube having an outwardly sloping wall in the vicinity of said jet,

and a cylindrical sleeve surrounding said out-- wardly sloping wall of the vapor tube whose outer surface is substantially parallel to the inner surface of the pump casing and forms the inner wall of the umbrella jet.

6. A vacuum diffusion pump comprising a casing, a vapor-conducting tube therein provided with an umbrella jet, said tube having a constricted portion so that the wall of the tube slopes outwardly as it approaches the jet, and a cylindrical sleeve surrounding the constricted portion of the vapor tube whose outer surface is substantially parallel to the inner surface of the pump casing and forms the inner wall of the umbrella jet.

7. A vacuum diffusion pump comprising a casing, a vapor-conducting tube therein having a constricted portion forming a throat-shaped passage, jet means on the vapor tube having an entrance through which vapor from the interior of the vapor tube passes into the jet means after it passes through said throat-shaped passage, said jet means being adapted to direct the vapor into the space between the vapor tube and the pump casing and in a reverse direction, the interior surface of the vapor tube sloping outwardly from said throat-shaped passage to the entrance to the jet means, and said jet means including a cylindrical sleeve surrounding the constricted portion of the vapor tube.

8. A vacuum difiusion pump comprising a casing, a vapor-conducting chimney therein, said chimney having a double-walled portion, the inner wall of said portion forming a throat-shaped passage, jet means on the chimney having an entrance through which vapor from the interior of the chimney passes into the jet means after it passes through said throat-shaped passage, said jet means being adapted to direct the vapor into the space between the chimney and the pump casing and in a reverse direction, the inner surface of the chimney sloping outwardly from said throat-shaped passage to the entrance to the jet means, and the outer wall of said doublewalled portion of the chimney being cylindrical and its outer surface constituting the inner wall of said jet means.

EARL W. FLOSDORF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS 

